Fix some bugs with high-alignment fields in continuations #981
Conversation
This file contains hidden or bidirectional Unicode text that may be interpreted or compiled differently than what appears below. To review, open the file in an editor that reveals hidden Unicode characters.
Learn more about bidirectional Unicode characters
The algorithm supports both assigning a fixed offset to a field prior to layout and allowing fields to have sizes that aren't multiples of their required alignments. This means that the well-known algorithm of sorting by decreasing alignment isn't always good enough. Still, we start with that, and only if that leaves padding around do we fall back on a greedy padding-minimizing algorithm. There is no known efficient algorithm for producing a guaranteed-minimal layout in all cases. In fact, allowing arbitrary fixed-offset fields means there's a straightforward reduction from bin-packing, making this NP-hard. But as usual with such problems, we can still efficiently produce adequate solutions to the cases that matter most to us. I intend to use this in coroutine frame layout, where the retcon lowerings very badly want to minimize total space usage, and where the switch lowering can indeed produce a header with interior padding if the promise field is highly-aligned. But it may be useful in a much wider variety of situations.
Previously, we would ignore alloca alignment when building the frame and just use the natural alignment of the allocated type. If an alloca is over-aligned for its IR type, this could lead to a frame entry with inadequate alignment for the downstream uses of the alloca. Since highly-aligned fields also tend to produce poor layouts under a naive layout algorithm, I've also switched coroutine frames to use the new optimal struct layout algorithm. In order to communicate the frame size and alignment to later passes, I needed to set align+dereferenceable attributes on the frame-pointer parameter of the resume function. This is clearly the right thing to do, but the align attribute currently seems to result in assumptions being added during inlining that the optimizer cannot easily remove.
|
@swift-ci Please test |
|
@swift-ci Please test macOS |
|
Linux test failure is unrelated; macOS is still having builder problems. |
|
@swift-ci Please test |
Sign up for free
to join this conversation on GitHub.
Already have an account?
Sign in to comment
Add this suggestion to a batch that can be applied as a single commit.
This suggestion is invalid because no changes were made to the code.
Suggestions cannot be applied while the pull request is closed.
Suggestions cannot be applied while viewing a subset of changes.
Only one suggestion per line can be applied in a batch.
Add this suggestion to a batch that can be applied as a single commit.
Applying suggestions on deleted lines is not supported.
You must change the existing code in this line in order to create a valid suggestion.
Outdated suggestions cannot be applied.
This suggestion has been applied or marked resolved.
Suggestions cannot be applied from pending reviews.
Suggestions cannot be applied on multi-line comments.
Suggestions cannot be applied while the pull request is queued to merge.
Suggestion cannot be applied right now. Please check back later.
Fixes rdar://56867744